The plume mode of convection - Mantle Convection for Geologists - page 86

Figure 7.7. Sequence from a numerical model in which a plume grows from a
thermal boundary layer. Panels show cross-sections of an axisymmetric plume.
The model is scaled to the mantle. A line of passive tracers marks fluid that
was initially in the thermal boundary layer. The spiral structure is due to thermal
entrainment into the head (see text). The viscosity is temperature dependent, the
value at the ambient mantle temperature being 10 22 Pa s. The bottom boundary
is maintained at 430 ◦ C above the interior temperature, and the viscosity of the
hottest fluid is about 1% of the ambient viscosity.
'tail'. These results have been confirmed in other experiments and in numerical
models. An example of the head-and-tail structure from a numerical model is shown
in Figure 7.7.
The head-and-tail structure only occurs when the viscosity of the plume material
is less than that of the surrounding material. It comes about as follows. A minimum
head size is required before it can detach from the thermal boundary layer and
rise through the mantle. The rising head then forces a path through the higher-
viscosity surrounding mantle. Once a path is forged, low-viscosity fluid requires
only a relatively thin conduit to continue flowing after the head. The lower the
viscosity, the thinner the conduit may be. The effect is illustrated quantitatively in
Figure 7.8.
The spiral structure evident in Figure 7.7 comes about because of thermal entrain-
ment into the plume head. As the warm head moves up through ambient mantle, it
warms adjacent material by thermal conduction, forming a thermal boundary layer
around the head, as sketched in Figure 7.9. This makes the newly warmed material
buoyant. As the head moves up, its internal circulation wraps some of the thermal
boundary layer material into itself, next to the plume tail material. The internal
circulation occurs because the fluid flowing up the plume tail is the fastest-rising
fluid, and the outer parts of the plume head are resisted by the surrounding fluid,